The present invention relates generally to an air purification system that determines the optimal wavelength of microwaves that are emitted to desorb water based on the humidity level or temperature of air in the air purification system.
Indoor air can include trace amounts of contaminants, including carbon monoxide and volatile organic compounds such as formaldehyde, toluene, propanal, butene, and acetaldehyde. Absorbent air filters, such as activated carbon, have been employed to remove these contaminants from the air. As air flows through the filter, the filter blocks the passage of the contaminants, allowing contaminant free air to flow from the filter. A drawback to employing filters is that they simply block the passage of contaminants and do not destroy them.
Titanium dioxide has been employed as a photocatalyst in an air purifier to destroy contaminants. When the titanium dioxide is illuminated with ultraviolet light, photons are absorbed by the titanium dioxide, promoting an electron from the valence band to the conduction band, thus producing a hole in the valence band and adding an electron in the conduction band. The promoted electron reacts with oxygen, and the hole remaining in the valence band reacts with water, forming reactive hydroxyl radicals. When a contaminant adsorbs onto the titanium dioxide photocatalyst, the hydroxyl radicals attack and oxidize the contaminants to water, carbon dioxide, and other substances.
Water and contaminants compete for adsorption sites on the photocatalyst. As there is a much greater concentration of water than contaminants, water has a greater probability of occupying a given adsorption site on the photocatalyst. For example, there are thousands of ppmv for water vapor and much less than one ppmv for a contaminant. Additionally, water forms hydrogen bonds on the photocatalyst that are much stronger than the van der Waals forces that retain a contaminant on the photocatalyst. Water that adsorbs onto the photocatalyst blocks access of the contaminants to the photooxidation sites on the photocatalyst, inhibiting photooxidation of the contaminants.
Photocatalytic activity of the photocatalyst is maximized at about 5 to 30% relative humidity, most preferably at 15% relative humidity. As humidity increases from this range, there is a steep decrease in the photocatalytic rate. For example, at a relative humidity of 60%, the photocatalytic rate decreases by a factor of two.
Microwaves can be employed to maintain an optimal photooxidation rate of the contaminants in a humid atmosphere. Microwaves selectively desorb water molecules from the photocatalyst, freeing the photooxidation sites so they can absorb contaminants. However, different wavelengths and intensities of microwaves are effective at different humidity or temperature levels. Therefore, the optimal wavelength or intensity of microwaves can change as the humidity or temperature level changes.
Hence, there is a need in the art for a system that determines the optimal wavelength or intensity of microwaves to desorb water based on the humidity or temperature level in the air purification system.